As used throughout the specification and claims, the term "blower" means any suitable air delivery unit, including centrifugal blowers and axial flow fans.
U.S. Pat. Nos. 4,878,360 and 4,922,727, which are assigned to the same assignee as the present application, disclose a transport refrigeration system which includes a refrigerant compressor, an internal combustion engine, such as a Diesel engine, for driving the compressor, a condenser, a condenser blower, an evaporator, an evaporator blower, and an adjustable speed arrangement for the evaporator blower. The adjustable speed arrangement of the '360 patent includes a variable pulley, the pitch diameter of which is selected by a linear actuator. The adjustable speed arrangement of the '727 patent includes an electric clutch and a one-way clutch. As pointed out in these U.S. Patents, it is beneficial to be able to control the speed of the evaporator blower independently of the speed of the engine, as the air requirements of the served load are not always directly proportional to engine speed.
The speed of an internal combustion engine driving a refrigerant compressor is conventionally controlled to one of two speeds, called "high speed" and "low speed", such as 2200 RPM and 1400 RPM., for example, the selection of which is automatically made by a thermostat in response to the deviation of the temperature of the served load from the desired or set point temperature. When the thermostat senses that the temperature of the load in the served space has entered a predetermined range close to the desired or "set point" temperature, the engine speed is dropped from high to low speed. It is desirable for the speed of the condenser blower to also drop when the engine speed drops to low speed, as the refrigeration capacity is reduced. However, it may or may not be desirable for the speed of the evaporator blower to also drop, depending upon the characteristics of the served load. Thus, the evaporator blower may have a drive arrangement, such as taught by the aforesaid patents, which maintains the air flow constant regardless of whether the engine is operating at high or low speed. During defrost of the evaporator coil, no air circulation through the served load is desired at all, and it is common to close shutters located between the evaporator blower and the served space to prevent the air from the evaporator blower from entering the served space during defrost.
While the adjustable speed evaporator blower arrangements of the aforesaid patents provide the desired function of being able to select the speed of the evaporator blower independently of the engine speed, it would be desirable and it is an object of the present invention to provide an infinitely variable speed drive for the evaporator blower which is easier to set up, adjust and control, and which will be easier to maintain.